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Electronics - Physics Form 4 Coursework e-Content CDs

Background knowledge

In form one, we classified materials into conductors and insulators in the topic cells and simple circuits. In this topic we shall introduce another class of materials known as semiconductors.

Objectives

By the end of the lesson you should be able to:

-State the differences between conductors and insulators

-Define intrinsic and extrinsic semi-conductors

-Explain doping in semiconductors

-Explain the working of a p-n junction diode

-Sketch current voltage characteristic for a diode

-Explain the applications of diode in rectification





 






Introduction

The knowledge of electronics is widely applied in the manufacture of electronic devices like T.Vs, radios, computers mobile phones among others. Semi conductors are major components in the circuits which aid in the operation of these devices.

Conductors and insulators

Materials which have free electrons within their structure are called conductors. The animation below shows the structure of a good conductor and an insulator. Click on the word "conductor" and observe what happens ,repeat this with the "insulator" and observe what happens.

Observation

When the conductor is used to complete the gap, electrons flow causing current to flow. The bulb lights. When the insulator is used to complete the gap, the bulb does not light.

Explanation
All materials contain two bands where electrons are found. These are the valence band and the conduction band. The two are separated by a gap known as the forbidden gap. There are no electrons in the forbidden gap.

Insulators

In insulators, the forbidden gap is too wide such that the electrons require a lot of energy to be exited into the conduction band. Click on the play button of the animation below to see visualization on this.

Conductors

In conductors the valence and conduction bands overlap (the forbidden band doesn't exist). Electrons are free and mobile to conduct current. Play the animation below to observe the behavior of electrons in a conductor.

Semiconductors

In these materials the forbidden gap is not as wide as in insulators. At low temperatures, all the electrons are in the valence band. However at room temperature, some electrons gain thermal energy and overcome the forbidden gap. The materials then become a conductor. Examples are silicon and germanium. Click on the play button of the animation below to see how electrons behave in semiconductors.

Types of semi-conductors

There are two types of semiconductors.


  • Intrinsic semiconductors
  • Extrinsic semiconductors

Intrinsic semiconductors

Silicon has an atomic number of 14. Its electronic configuration is 2.8.4. The diagram below shows a silicon lattice structure.



Explanation.

At low temperatures all the electrons are held up in the covalent bonds. At high temperatures some electrons gain heat energy, the bond is broken and they move to the conduction band. The free electrons move freely within the lattice and are available for conduction. A free electron leaves a positive space referred to as a hole. This creates instability by attracting electrons from the neighboring atoms causing a net movement of electrons. Play the animation below to observe this. When the material is connected to a circuit the electrons flow in one direction making the hole to drift in the opposite direction.

Conduction in intrinsic semiconductors

The animation below shows how electrons in an intrinsic semiconductors behave when in an electric circuit. Click on its play button and make your observations.

Explanation

A material which increases its electrical conductivity from within is called intrinsic semiconductor. In this material, the number of free electrons is the same as the number of holes created.

Extrinsic semiconductors


These are materials whose conductivity is increased by adding small amounts of impurities into the pure material. The process is called doping. The materials which are doped are known as extrinsic semiconductors. There are two types of extrinsic semi conductors namely:

a) P-type semiconductors

b) N-type semiconductors

P-type semiconductors

This is obtained by doping the material with a group 3 element like aluminium, boron and gallium. The animation below shows how the semiconductor is doped. Click on the [dope] button and observe what happens.

Observations
When the {dope] button is clicked a hole is created which makes electrons to jump from one position to another.

N-type semiconductor

This is obtained by doping a semiconductor with a group 5 element like phosphorus, antimony, arsenic among others. The animation below shows how the semiconductor is doped. Click on the {dope} button and observe what happens.

Observation

A phosphorus atom moves into the structure providing a free electron.

Explanation

The phosphorus atom introduces a free electron in the structure which makes the structure to become a conductor.

The Junction diode.

A p-type semi conductor can be joined to an n-type semiconductor of the same material to form a p-n junction diode. Once the junction is formed, the holes in the p-type near the junction move towards the n-type side and the electrons in the n-type near the junction cross over towards the p-type side. The movement causes neutralization around the junction. This creates a barrier there which prevents further movement of charges. This barrier is called the depletion layer. The animation below shows how a depletion layer is formed. Click "next" to view a visualization on this.

Click on the play button to make your observations.

Forward biasing

This is connecting a p-n junction diode to a p.d in such a way that the p-type is connected to the positive terminal and the n-type to the negative terminal. The animation below shows the forward bias connection.

Observation
The negative charges move towards the p-type side and the positive charges towards the n-type side. Current flows and the bulb lights.
Explanation
The positive terminal of the cell repels the holes towards the n-type side and attracts the electrons. The negative terminal of the cell repels the electrons from the n-side and attracts the holes. This reduces the size of the depletion layer making it easy for charges to cross over and current flows.

Forward bias characteristic

The sketch below is an illustration showing the I-V relationship.

Reverse biasing

This is connecting a p-n junction to a p.d in such a way that the p-type is connected to the negative terminal and the n-type to the negative terminal. The animation below shows reverse bias connection. Play it and make your observations.

Observation

The negative charges move to the right and the positive towards the left widening the depletion layer. The bulb does not light.

Explanation

The holes are attracted by the negative terminal of the cell while the negative charges are attracted to the positive terminal of the cell. This widens the depletion layer increasing resistance hence no current flows.

Applications of diodes

Rectification

This is transformation of an alternating current into a direct current. A diode is used in rectification because of its property of exhibiting high resistance when reverse biased and low resistance when forward biased. There are two ways of rectifying an alternating current.

a) Half wave rectification

b)Full wave rectification

An a.c connected to a C.R.O displays a graph a shown in the visualisation. This gives the form of the wave to be rectified. Click "next" to view a visualization of this.

Click on the play button to make your observations.

a) Half wave rectification


In half wave rectification, a diode is connected to the circuit. In the first half cycle of the a.c. the diode becomes forward biased and allows the current to flow through. In the reverse cycle, the diode becomes reverse biased and cuts off the current flow and the second part of the curve on the C.R.O. The animation below shows how a half wave rectification is done. Click on the play button and observe what happens.


Observation

Current in the first half cycle is seen to flow but the second half cycle is cut off. A half wave is displayed on the C.R.O.

Full wave rectification

There are two ways of achieving this connection, either using two diodes with a centre tapped transformer or using four diodes.

a) Using two diodes:

The animation below shows how an a.c can be rectified using two diodes. Play it and make your observations.


Observation

When the switch is closed, half waves are displayed on the screen.

Explanation

In the first half cycle, current flows through C, A, D1, RL, and back to C. A half wave is displayed on the C.R.O. In the second half cycle, current flows through C, D2, B, RL and back to C. the second half wave is displayed on the C.R.O. The process continues and many half waves are displayed.

Using four diodes

The animation below shows how four diodes can be used in full wave rectification. Click on the play button to observe how this is done


Observation

When the switch is closed, half waves are displayed on the screen of the C.R.O.

Explanation

When the switch is closed in the first half cycle, current moves from source to D4,RL,D3and back. A half cycle is seen in the C.R.O. In the second half cycle, current moves to D2, RL, D3, and back to the source. Another half cycle is displayed on the screen. The process is repeated and a series of half waves is seen on the C.R.O.

Using four diodes

The animation below shows how four diodes can be used in full wave rectification. Click on the play button to observe how this is done.

Observation

When the switch is closed, half waves are displayed on the screen of the C.R.O.

Explanation

When the switch is closed in the first half cycle, current moves from source to D4,RL,D3 and back. A half cycle is seen in the C.R.O. In the second half cycle, current moves to D2, RL, D3, and back to the source. Another half cycle is displayed on the screen. The process is repeated and a series of half waves is seen on the C.R.O.

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